Conventional wave energy converters have experienced considerable problems including low efficiency, complex design, and/or high cost of maintenance resulting in considerable limitations in their use.
Firstly, known WECs use mainly the vertical component of the wave motion. Since the wave motion is generally circular, a significant component of the motion is wasted, see for example the devices in U.S. Pat. No. 4,453,894, U.S. Pat. No. 6,857,266, WO2004065785, U.S. Pat. No. 4,232,230, U.S. Pat. No. 4,672,222, U.S. Pat. No. 5,411,377, and the devices Pelamis, AWS WEC and AquaBuOY marketed respectively by the companies OPD Ltd (www.oceanpd.com), AWS Ocean Energy Ltd (www.waveswing.com) and AquaEnergy Group Ltd (www.aquaenergygroup.com).
In some cases, e.g. the devices shown in U.S. Pat. No. 4,453,894 and U.S. Pat. No. 6,857,266, the installation has a natural frequency of oscillation, and therefore is capable of using efficiently only waves with certain frequencies, or a latching mechanism is needed to overcome this limitation.
On the other hand, in order to seek the highest possible efficiency, known WECs using floating or near surface elements need to be oriented depending on the direction of the wave system which is to be harvested for energy. In any case, the efficiency is satisfactory only when harvesting monochromatic waves, or in any case waves moving all in the same general direction, as in the device of US2005167988A1, and in the device Pelamis and WavePlane (by the company WavePlane Production A/S, www.waveplane.com).
Furthermore, many known devices must be linked to the ground or placed in the surf zone, like for example in US2005167988A1, U.S. Pat. No. 5,411,377, and in the above mentioned devices AWS WEC, Pelamis and AquaBuOY. Therefore, they cannot be positioned where the waves are biggest and with the highest energy potential. Moreover, the on-shore or near-shore ones are bulky and hardly compatible with environmental protection.
Finally, as mentioned, the majority of the known devices use a surface float to extract energy from the wave motion or in any case they have a substantial part of their volume above the surface of the water at least during some part of the wave cycle, which can expose them excessively to severe weather and to drag due to wind and small waves. This is the case of the installations shown in U.S. Pat. No. 4,453,894, U.S. Pat. No. 6,857,266, WO2004065785, U.S. Pat. No. 4,232,230, U.S. Pat. No. 4,672,222, U.S. Pat. No. 5,411,377, and the Pelamis, AquaBuOY, WavePlane and Wave Dragon (by the company Wave Dragon ApS, www.wavedragon.net). Due to the presence of surface elements which have necessarily a limited size, there is also a structural limitation to the amount of energy which a single device can harvest.